Alexithymia and reactive aggression: The role of the amygdala

Highlights

• Alexithymia is associated with reactive aggression, but not proactive aggression.

• Alexithymia, reactive aggression, and amygdala volume are positively associated.

• Controlling for amygdala volume, alexithymia is unrelated to reactive aggression.

• Amygdala volume partly explains the alexithymia-reactive aggression relationship.

Abstract

Past research suggests an association between reactive aggression and alexithymia, but neural mechanisms underlying this association remain unknown. Furthermore, the relationship between proactive aggression and alexithymia remains untested. This study aimed to: (1) test whether alexithymia is more related to reactive than proactive aggression; and (2) determine whether amygdala, insula, and/or anterior cingulate cortical (ACC) volume could be neurobiological mechanisms for this association. One hundred and fifty-six community males completed the Reactive-Proactive Aggression Questionnaire and the Toronto Alexithymia Scale. Amygdala, insula, and ACC volumes were assessed using MRI. Alexithymia was positively associated with reactive but not proactive aggression. Alexithymia was positively and bilaterally associated with amygdala and anterior cingulate volumes. Reactive aggression was positively associated with right amygdala volume. Controlling for right amygdala volume rendered the alexithymia-reactive aggression relationship non-significant. Results suggest that increased right amygdala volume is a common neurobiological denominator for both alexithymia and reactive aggression. Findings suggest that greater right hemisphere activation may reflect a vulnerability to negative affect, which in turn predisposes to experiencing negative emotions leading to increased aggression. Findings are among the first to explicate the nature of the alexithymia-aggression relationship, with potential clinical implications.

Introduction

Research on aggression often distinguishes between reactive and proactive aggression (Teten et al., 2008). Reactive aggression (RA), also called “impulsive aggression”, is associated with high emotional arousal and anxiety (Bubier and Drabick, 2009, Raine et al., 2006), and usually occurs as an impulsive response to a perceived threat or provocation. Proactive aggression (PA), also called “instrumental aggression,” is generally planned in advance and motivated by the anticipation of a reward (Euler, Steinlin and Stadler, 2017). These two constructs are highly related (Raine et al., 2006), and share common correlates such as drug use and parental violence (Connor et al., 2003). However, there are also unique risk factors associated with each form of aggression. Whereas hyperactivity, low IQ, and higher attentional bias towards aggression-related words have been linked to RA (Brugman et al., 2015, Connor et al., 2003), PA appears to be associated with lower levels of cognitive and affective empathy, as well as lower attentional bias towards aggression-related words (Brugman et al., 2015, Euler et al., 2017). Given the conceptual differences between RA and PA, as well as their distinct correlates, findings about one form of aggression may not be generalizable to the other.

One phenomenon that has been studied in relation to aggression is alexithymia. Alexithymia, a term first used by Sifneos (1973), refers to a psychological construct characterized by difficulty in understanding, identifying, and expressing one's own emotions (Nemiah, 1977). Research has suggested that alexithymia is related to a wide range of psychiatric conditions, including depression (Honkalampi et al., 2000), anxiety (Berardis et al., 2006), psychosis (van der Velde et al., 2015), and alcohol dependence (Thorberg et al., 2009). The small body of research examining the relationship between alexithymia and aggression has found alexithymia to be associated with increased aggression (Velotti et al., 2016). However, these studies have either looked at RA exclusively or failed to distinguish between RA and PA. This leaves the relationship between PA and alexithymia unexplored.

Examining the relationship between alexithymia and each form of aggression may shed light on both the causes of aggression and the nature of alexithymia. If alexithymia is related to PA as well as RA, it might suggest that both forms of aggression stem from an emotional disturbance, despite the common conception of RA as being the “emotional” form of aggression and PA as being unrelated to emotion. On the other hand, if alexithymia is only related to RA, it would suggest that understanding and identifying one's own emotions is important for behavioral inhibition and regulation, given that RA tends to be impulsive and driven by unregulated emotion (Vitiello and Stoff, 1997).

There is reason to believe that alexithymia may be exclusively related to RA. Past research has indicated that alexithymia is associated with impulsivity (Velotti et al., 2016), which itself is related to RA (Connor et al., 2003, Hecht and Latzman, 2015). PA aggression, on the other hand, is generally premeditated, so the link between alexithymia and impulsivity does not support the idea of a link between alexithymia and PA. Research has also suggested that while individuals with alexithymia have less understanding of their own emotions, they experience a greater number of emotions, particularly negative ones, and experience them more intensely (Leising et al., 2009). The heightened experience of negative affect in alexithymia may even be due to the lack of understanding of emotional feelings. Since RA is associated with high emotional arousal, and especially with negative affect (Blair, 2001), one might expect that a condition associated with more negative emotions and greater emotional intensity would lead to more RA.

Understanding the neural basis of the relationship between alexithymia and aggression could improve our understanding of the driving factors behind aggressive behavior, and potentially help researchers and clinicians reduce aggressive behavior in alexithymic patients. The amygdala is known to play a central role in human emotion, particularly negative emotions (Hamann et al., 2002). Indeed, prior studies have generally found smaller amygdala volume and reduced amygdala response to negative stimuli to be associated with both RA (Pardini et al., 2014) and PA (Lozier et al., 2014, Pardini et al., 2014), as well as with alexithymia (Goerlich-Dobre et al., 2015, Ihme et al., 2013). However, these findings are not entirely consistent. Higher alexithymia scores have been positively correlated with the strength of amygdala connectivity to other brain regions when participants process movies portraying anger (Hadjikhani et al. 2017), in line with the notion that alexithymia, or at least scores on the TAS-20, are associated with increased experiencing of negative emotions (Leising et al., 2009). The reason for these inconsistencies are unclear, but may be due to small sample sizes as many of these studies used fewer than 100 participants (Lozier et al., 2014, Pardini et al., 2014). They may also be due to inconsistencies in the use of the left, right, or bilateral amygdalae, or in the choice of behavioral measurements. While the direction of findings remains to be clarified, the amygdala may serve as a common factor that contributes to both alexithymia and aggression. Because the amygdala plays such a key role in emotion, it may be that amygdala abnormalities result in an increased tendency to engage in aggressive behavior, as well as difficulties in understanding one's own emotions.

While the amygdala is well-known as part of the neural circuitry for emotion and thus is a prime candidate for being a neural correlate of alexithymia and aggression, two other brain regions have been implicated in alexithymia and aggression: the insula and the anterior cingulate cortex (Dambacher et al., 2015, Goerlich-Dobre et al., 2014, Kano and Fukudo, 2013). Like the amygdala, the insula has been implicated in emotional processing and experiences, though prior research has suggested differential activation between these brain regions based upon the specific emotion (e.g., amygdala – fear, insula – disgust; Philips et al., 2004). The anterior cingulate cortex (ACC), on the other hand, is thought to be involved in the monitoring and regulation of subcortical structures such as the amygdala and insula as well as error monitoring and reappraisal (Bush et al., 2000, Van Veen et al., 2001). These two regions have been thought to partly underlie alexithymia via abnormal neurotransmitter concentrations (GABA in the ACC and glutamate in the insula) that contribute to disrupted interoceptive awareness (i.e., the perception of internal bodily sensations and changes) (Ernst et al., 2013). Moreover, both the anterior cingulate cortex and the insula have been found to be involved in reactions to provocative situations, thus impairments in these regions may contribute to aggression, particularly RA. As such, these two regions may also be a common neural mechanism that underlie both alexithymia and aggression.

The present study examines the relationships between alexithymia and both RA and PA, while considering the role of amygdala volume. It was hypothesized that alexithymia would be more related to RA and PA aggression and that amygdala volume as the primary neural candidate would account for this association. Given mixed prior findings on the amygdala and alexithymia (Hadjikhani et al. 2017) and prior findings of heightened negative affect experiences in those with alexithymia (Leising et al., 2009), the direction of effect regarding amygdala volume and alexithymia was left as exploratory. As secondary, exploratory analyses, the insula and ACC were also examined as other brain regions that may underlie the alexithymia – aggression relationship.